The present invention relates to an ionization chamber having two electrodes with a variable spacing and with a radioactive source for the ionization of the electrode gap, particularly for use in an ionization smoke detector.
Examples of ionization smoke detectors of the type involved here are described, for example, in the U.S. Pat. Nos. 3,710,110, issued Jan. 9, 1973 and 3,767,917, issued Oct. 23, 1973, both to Lampart et al.
In general, known ionization smoke detectors have two series-connected ionization chambers with different smoke sensitivities. For example, one of the chambers, normally called the measuring ionization chamber, is made extensively accessible to air, while the other chamber, normally called the reference ionization chamber, is essentially sealed against the atmosphere or screened against air access. In such ionization smoke detectors, use is made of the fact that when heavier particles, e.g. of smoke, penetrate into the chamber, the stream of atmospheric ions formed by the radioactive source and which flows between the electrodes is reduced. As a result, the chamber resistance rises. As the reference ionization chamber is not or only slightly influenced by smoke, if at all, its stream of ions remains virtually constant, particularly when in the saturation range. Therefore, when the voltage drop in the measuring ionization chamber increases upon penetration of smoke into the chamber, an evaluation circuit connected to the chamber gives an alarm signal after its voltage drop has exceeded a predetermined threshold value.
In practice, it is often necessary to be able to modify the threshold value, and consequently the sensitivity, of such an ionization smoke detector to adapt it to ambient conditions. This can be brought about electrically on the one hand by modifying the evaluation circuit and on the other by varying the stream of ions or the resistance of one of the two ionization chambers.
Various ionization smoke detectors are already known in which the stream of ions or the resistance of either the measuring ionization chamber or the reference ionization chamber is modified by changing the spacing of the two electrodes.
For example, the British Pat. No. 1,446,780 to Gacogne, published Aug. 18, 1976 and the Australian Pat. No. 402,078 to Ashwin published Apr. 26, 1968 describe detectors with an ionization chamber in which the electrode spacing may be adjusted by means of an adjusting screw. The British Pat. No. 1,088,976 published Oct. 25, 1967 discloses a detector with an ionization chamber in which the electrode spacing may be adjusted and fixed by means of a locking screw which, however, is not accessible from outside the detector.
When changing the sensitivity of an ionization smoke detector, preference is given to changing that of the reference ionization chamber, because in this case there is no need to influence the geometrical conditions, and consequently the smoke sensitivity, of the measuring ionization chamber.
In known ionization chambers such a modification to the electrode spacing is generally brought about by fixing one electrode to a screw which is passed through the rigid chamber casing and which can be turned from the rear wall of the chamber. However, such an adjustment by means of a simple screw thread has the disadvantage that over a period of time, and particularly under the action of vibrations or shocks, the setting changes by itself. Thus, a smoke detector equipped with such an ionization chamber is not operationally reliable over a period of time, unless the adjusting screw is locked, e.g. with a thread setting compound. As a result, once it is locked, the sensitivity cannot be readily adapted to other conditions. In other known ionization chambers with spacing adjustment, only a small electrode plate is placed on the adjusting screw, obviously for stability reasons. Thus, the change to the stream of ions which can be brought about by varying the electrode gap is much smaller than in the case of larger electrode dimensions and can in no way be considered optimum. A further disadvantage is that such adjustment mechanisms require a large amount of space outside the ionization chambers and can therefore undesirably increase the overall dimensions of an ionization smoke detector.